Wireless power transfer in a room occupied by humans is an unsolved problem. Current technologies available for wireless energy transfer can include: Inductive coupling, omnidirectional electromagnetic radiation, unidirectional electromagnetic radiation, and magnetic resonance coupling. Inductive coupling is a low cost solution with high efficiency, but the transmission distance is less than the size of the inductive coil. Magnetic resonance coupling provides greater range, up to several meters, but may damage biological tissue in the vicinity. Omnidirectional radiation can transfer power to very small receivers, but the power density decreases rapidly as a function of the distance between sender and receiver. Directed radiation provides high efficiency and range, but the sender needs to track the position of mobile receivers.
Power transfer by directed radiation can be done by microwave transmission and optical transmission. Optical power transmission using lasers has the main advantage of allowing much smaller apertures for both the transmitter and the receiver, however depends on good weather. With microwaves, large power densities are achievable in resonant cavities and waveguides, such as within microwave ovens. Microwave power transfer can have an efficiency over 80%. However, outside of a confined waveguide or cavity, intense microwave radiation is harmful to living beings and make a commercial deployment of microwave power transfer impossible. For the same reason, it is difficult to transmit microwaves through dense media. This has implications for communications with and supplying power to underwater craft and embedded sensors, such as medical sensors inside the human body.
While the present invention is susceptible to various modifications and alternative forms, exemplary embodiments thereof are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description of exemplary embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the embodiments above and the claims below. Reference should therefore be made to the embodiments above and claims below for interpreting the scope of the invention.
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